Third generation (3G) mobile radio standards (e.g., CDMA2000, Universal Mobile Telecommunications Systems (UMTS)) for wireless communication systems are the result of a massive worldwide effort involving many companies since the mid-1990s. 3G standards initially supported data rates up to 2 megabits per second (Mbps) but have evolved to support data rates up to 14 Mbps.
FIG. 1 is a diagram of an example prior-art wireless communication system 100 consisting of a wireless base station 110 and a plurality of wireless mobile devices (e.g., a wireless handset 115, a laptop computer 116, a personal digital assistant (PDA) 117, etc.) with which the wireless base station 110 is capable of communicating data and/or voice information.
FIG. 2 is a block diagram illustration of an example prior-art manner of implementing the wireless handset 115 of FIG. 1. The wireless handset 115 contains an antenna 210 used to transmit and receive wireless radio frequency (RF) signals to and from the wireless base station 110 (not shown). To process the RF signals received from the wireless base station 110 via the antenna 210, and to generate RF signals for transmission to the wireless base station 110 via the antenna 210, the example wireless handset 115 contains a RF transceiver 215. The RF transceiver 215 modulates baseband transmit signals received from the analog baseband processor 220 to RF band signals, and demodulates RF band signals received from the RF transceiver 215 to baseband.
To handle conversion from the analog domain to the digital domain, the example wireless handset 115 further includes an analog baseband processor 220. The analog baseband processor 220 comprises an analog-to-digital (A/D) converter (not shown) to transform analog baseband signals received from the RF transceiver 215 into digital baseband signals for the digital baseband processor 225. The analog baseband processor 220 also includes a digital-to-analog (D/A) converter (not shown) to transform digital baseband signals received from the digital baseband processor 225 into analog baseband signals for the RF transceiver 215.
To implement the digital receive functions (e.g., equalization, despreading, demodulation, etc.) and the digital transmit functions (e.g., modulation, spreading, etc.) the example wireless handset 115 includes the digital baseband processor 225. To encode and decode signals representative of speech, the example wireless handset 1 5 further includes a voice-coder-decoder (vocoder) 230. The vocoder 230 comprises a speech encoder (not shown) to translate digital samples representing speech spoken by the user (not shown) of the wireless handset 115 into a stream of digital data to be processed for transmission to the wireless base station 110 by the digital baseband processor 225, the analog baseband processor 220, the RF transceiver 215, and the antenna 210. Likewise, the vocoder 230 comprises a speech decoder (not shown) to translate a stream of digital data received from the wireless base station 110 into digital samples representative of speech to be listened to by the user of the wireless handset 115.
The example wireless handset 115 further comprises a voice transceiver 235 that implements conversion of analog signals representative of speech received from a microphone 245 into digital samples using, among other things, an A/D converter (not shown). The voice transceiver 235 further implements conversion of digital samples representative of speech received from the vocoder 230 to analog signals which can be played out a speaker 240 using, among other things, a D/A converter (not shown).
Example implementations of the antenna 210, the RF transceiver 215, the analog baseband processor 220, the vocoder 230, the voice transceiver 235, the speaker 240, and the microphone 245 are well known to persons of ordinary skill in the art, and, thus, will not be discussed further.
Asymmetric user services (e.g., web browsing) requiring high downlink capacity (i.e., from wireless base station to the wireless mobile device) led to the development of the High Speed Downlink Packet Access (HSDPA) and the Evolution Data and Voice (EV-DV)/Evolution Data Optimized (EV-DO) standards. Efficient downlink wireless receivers that can operate in the presence of multiple transmission paths are important to achieving high downlink capacity. In subsequent discussions the term multipaths refers collectively to a plurality of transmission paths by which a signal transmitted by one device is received by a second device. In such circumstances, a receiver receives the transmitted signal a number of times wherein each received version of the transmitted signal has a different time delay, signal attenuation, and phase relative to the other received versions. Multipaths are created by reflections of the wireless signal off of objects located near the transmitter, the receiver, or in between. For example, buildings, bridges, cars, clouds, etc. Further, the term multipath refers to one of the plurality of transmission paths between the first device and the second device (i.e., one of the paths comprising the multipaths).
It is widely recognized that the well-known, conventional RAKE receiver suffers significant degradation in performance in the presence of multipaths on the downlink of a wireless communication system. In 3G standards, code transmissions on the downlink are typically designed to be orthogonal at a transmitter, but multipaths destroy the orthogonality resulting in significant inter-code interference at a wireless receiver. In the context of 3G, orthogonality refers to the use of spreading sequences that are orthogonal to each other, i.e., there is no correlation between a first spreading sequence and a second spreading sequence. The lower the CDMA spreading factor being utilized and the closer a user is located to the wireless base station 110, the more deleterious the effect of interference due to multipaths becomes. High speed data wireless systems such as EV-DV/EV-DO and HSDPA for UMTS employ a low spreading factor and may utilize a scheduling rule that tends to select users that are close to the wireless base station 110. The RAKE receiver does little to mitigate the effects of the multipath interference, therefore, the effects of multipath interference on these wireless mobile devices is quite significant.
Wireless receivers that try to mitigate the multipath interference in some fashion are termed advanced receivers. Two broad classes of CDMA advanced wireless receivers are equalizers and interference cancellers. If a received chip level signal, which has been distorted by multipaths, is sufficiently equalized prior to correlation with a spreading code, there is only a single path present during a subsequent despreading operation. A chip represents a time duration of a CDMA signal corresponding to one value in a pseudo-noise (PN) code sequence used to spread/de-spread the CDMA signal. Assuming orthogonal spreading sequences, an equalizer can largely restore orthogonality of multiple codes that started out orthogonal at the transmitter even in the presence of multiple transmission paths. Implementing equalization of chip level signals, rather than symbol level signals, allows the application of equalization to wireless communication systems utilizing long spreading sequences.
A downlink of a CDMA system typically uses a broadcast common pilot channel (CPICH) to transmit CPICH symbols that are known a priori to the wireless mobile device. The CPICH is used by a receiver in a wireless mobile device for synchronization, channel estimation, handoff support, etc. Additionally, in the case of an equalizer, the CPICH symbols may be used to train an equalizer. Frequently the normalized least mean squares (NLMS) algorithm is used to adapt equalizer coefficients. For simplicity, we shall refer to such CPICH trained NLMS equalizers as “CPICH based NLMS”.
FIG. 3 is a block diagram illustration of an example prior-art manner of implementing the digital baseband processor 225 of FIG. 2. The digital baseband processor 225 generally includes a transmitter 310 and a receiver 315. To convert digital data representative of encoded speech received from the vocoder 230 into CDMA transmit signals, the transmitter 310 includes a channel encoder 320, a modulator 322, and a spreader 325. The channel encoder 320 includes, among other things, the following functions: channel encoding, cyclic redundancy check (CRC) generation, conversion to blocks of data, rate matching, interleaving, multiplexing, etc. The modulator 322 modulates (e.g., using QPSK, QAM, etc.) the output of the channel encoder 320 which is then multiplied with a PN code sequence by the spreader 325 to create a digital CDMA transmit signal. The PN code sequence is formed as an exclusive-or of a cell specific PN code sequence and a channel specific PN code sequence.
To help mitigate the effects of multipath interference, the example receiver 315 includes an equalizer 328 to apply a filter to an input chip level signal 326. As discussed above, the equalizer 328 works to restore orthogonality of signals being received by the receiver 315. Filter coefficients of the equalizer 328 are adapted using an error signal 331 formed as a difference between a spread pilot signal 332 and an output 329 of the equalizer. The spread pilot signal 332 is formed by multiplication of CPICH symbols and a first PN code sequence. The CPICH symbols are those transmitted by the wireless base station 10 on the CPICH. The first PN code sequence is an exclusive-or of the cell specific PN code sequence and a CPICH specific code sequence. Because the duration of each CPICH symbol is multiple (e.g., 256) chips, the multiplication of the CPICH symbols and the first PN code sequence multiplies each CPICH symbol by N chips of the first PN code sequence, where N is the duration of each CPICH symbol.
If the equalizer 328 adapts its filter coefficients using NLMS, then the equalizer update configuration shown in FIG. 3 (comprising the equalizer 328, the equalizer output 329, the spread pilot signal 332, and the error 331) is the conventional, prior-art “CPICH based NLMS.” In a conventional “CPICH based NLMS” chip level equalizer, the input chip level signal 326 used in equalizer training comprises not only multipath signals of the CPICH, but also signals from one or more other downlink channels. At a chip level these signals are all roughly the same power, thus, the variance of the error signal 331 used in equalizer training will be quite large. Such a large variance limits the achievable performance of the equalizer 328.
Because each CDMA transmit signal has been spread by multiplication with a second PN code sequence (formed as an exclusive-or of the cell specific PN code sequence and the data channel specific code sequence) in the wireless base station 110, the receiver 315 includes a despreader 330 that correlates the equalized received signal 329 with the second PN code sequence. This second PN code sequence is the same PN code sequence used in the wireless base station 10 to spread data symbols currently being received. Outputs of the correlation process are provided every spreading factor (SF) chips. If adequate equalization is not performed in the presence of multipaths, every significant multipath must be individually despread by the despreader 330.
The outputs of the despreader 330 are passed through a demodulator 335 before being passed to a channel decoder 340 that performs, among other things: de-multiplexing, de-interleaving, rate detection and de-rate matching, conversion from blocks, CRC checking, channel decoding, etc.
Example implementations of the channel encoder 320, the modulator 322, the spreader 325, the equalizer 328, the despreader 330, the demodulator 335, and the channel decoder 340 are well known to persons of ordinary skill in the art, and, thus, will not be discussed further.